Contamination barrier and lithographic apparatus comprising same

ABSTRACT

A rotatable contamination barrier for use with an EUV radiation system is disclosed. The contamination barrier has a blade structure configured to trap contaminant material coming from a radiation source, a bearing structure, coupled to a static frame, configured to rotatably bear the blade structure, and an eccentric mass element displaced relative to a central axis of rotation to balance the blade structure in the bearing structure.

FIELD

The present invention relates to a contamination barrier and alithographic apparatus comprising same.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive metal compound (resist) provided on the substrate.In general, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

Radiation sources used in EUV lithography typically generate contaminantmaterial that is harmful to the optics and the working environmentwherein the lithographic process is carried out. Such is especially thecase for EUV sources operating via a laser induced plasma or dischargeplasma. Hence, in EUV lithography, a desire exists to limit thecontamination of the optical system arranged to condition the beam ofradiation coming from an EUV source. To this end, a foil trap, forinstance, as disclosed in European patent application publicationEP1491963, has been proposed. A foil trap uses a high number of closelypacked foils or blades. Contaminant debris, such as micro-particles,nano-particles and ions can be trapped in the walls provided by theblades. Thus, the foil trap functions as a contamination barriertrapping contaminant material from the source.

SUMMARY

In an embodiment, a rotatable foil trap may be oriented with an axis ofrotation oriented along an optical axis of the system, in particular infront of an extreme ultraviolet radiation source configured to provideextreme ultraviolet radiation. The blades configured to trap contaminantmaterial thus may be radially-oriented relative to a central rotatingshaft of the contamination barrier and may be aligned substantiallyparallel to the direction of radiation. By rotating the foil trap at asufficiently high speed, traveling contaminant debris may be captured bythe blades of the contaminant barrier. Due to design limitations, therotation speed of the contaminant barrier may be quite high, sinceotherwise the length of the blades along the direction of travel of thedebris would be unacceptably large. Typical revolution speeds are15000-30000 RPM. Furthermore, the foil trap is operated in (near) vacuumconditions, which gives special constraints to the type of bearing thatcan be used for the rotating foil trap. In particular, one type ofbearing that may be used is a gas bearing, wherein the rotating shaft ofthe foil trap is borne by gas (e.g., air) pressure. To not compromisethe vacuum conditions, the gap present between the shaft and such abearing should be kept very small, typically only several microns. Aconsequence of such an arrangement is that there may be imbalance of thefoil trap. Such imbalance may be detrimental to the vacuum and/or to theoperation of the machine.

It is desirable, for example, to provide a rotatable contaminationbarrier that has improved balancing properties.

According to an aspect of the invention, there is provided a rotatablecontamination barrier for use with an EUV radiation system, the barriercomprising:

a blade structure configured to trap contaminant material coming from aradiation source;

a bearing structure, coupled to a static frame, configured to rotatablybear the blade structure; and

an eccentric mass element displaced relative to a central axis ofrotation to balance the blade structure in the bearing structure.

According to an aspect of the invention, there is provided a balancingunit to balance a rotatable contamination barrier, the unit comprising:

a bearing structure configured to bear a blade structure of therotatable contamination barrier;

a imbalance sensor unit configured to provide a signal of a sensedimbalance of the blade structure in the bearing structure duringrotation of the blade structure; and

a calculating unit configured to calculate a location to provide one ormore eccentric mass elements on or to the blade structure and the amountof such mass, the calculating unit communicatively coupled to theimbalance sensor.

According to an aspect of the invention, there is provided a method ofbalancing a rotatable contamination barrier for use in an EUV radiationsystem, comprising:

bearing a blade structure in a bearing structure provided in a vacuumenvironment;

sensing an imbalance of the blade structure in the bearing structureduring rotation of the blade structure; and

calculating, based on the sensed imbalance, a location to provide aneccentric mass element on or to the blade structure and an amount ofsuch mass.

According to an aspect of the invention, there is provided a method ofcleaning a rotatable contamination barrier for use with an EUV radiationsystem, comprising:

adjusting an eccentric mass element of a blade structure of thecontamination barrier to provide an imbalance; and

rotating the blade structure with the imbalance to shake clean thecontamination barrier.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

a rotatable contamination barrier configured to receive a beam ofradiation, the contamination barrier comprising a blade structureconfigured to trap contaminant material coming from a radiation source,a bearing structure, coupled to a static frame, configured to rotatablybear the blade structure, and an eccentric mass element displacedrelative to a central axis of rotation to balance the blade structure inthe bearing structure;

an illumination system configured to condition the radiation beam;

a support constructed to support a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate; and

a projection system configured to project the patterned radiation beamonto a target portion of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 shows a general setup of a rotating foil trap according to anembodiment of the invention;

FIG. 3 shows a schematic axial cross-sectional view of the rotating foiltrap according to FIG. 2;

FIG. 4 shows a schematic lateral cross-sectional view of the rotatingfoil trap according to FIG. 3;

FIG. 5 shows schematically an exploded view of a balancing unitaccording to an embodiment of the invention; and

FIG. 6 shows an embodiment of an automatic balancing mechanism.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or EUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator and acondenser. The illuminator may be used to condition the radiation beam,to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF2 (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor IF1 canbe used to accurately position the patterning device MA with respect tothe path of the radiation beam B, e.g. after mechanical retrieval from amask library, or during a scan. In general, movement of the supportstructure MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the support structure MT maybe connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Although the principles of one or more embodiments of the invention maybe applied to a rotatable contamination barrier having any rotatableblade structure, FIG. 2 schematically shows an exemplary embodiment of arotatable contamination barrier or foil trap 1 wherein the bladestructure 2 is comprised of a central rotation shaft 3 with blades orfoils 4 mounted thereon. The barrier 1 is typically used in or with aradiation system 5 to provide a projection beam of radiation. In anembodiment, the radiation system 1 comprises an extreme ultravioletradiation source 6 configured to provide extreme ultraviolet radiation.In FIG. 2, the dashed lines represent EUV radiation 7 coming from EUVsource 6, typically a laser induced plasma source or a plasma dischargesource such as a tin, lithium or xenon source, however, other sourcesare possible, in particular, any other source that produces EUVradiation in combination with fast particles that escape from the source6 and that should be trapped in order to prevent damage to thedownstream optics of the lithographic apparatus (not shown). To thisend, the blade structure 2 is provided with a plurality of closelypacked blades 4 configured to trap contaminant material coming from theradiation source 6. In the exemplary embodiment, the blades 4 areradially oriented relative to a central rotation shaft 3 of thecontamination barrier 1. By rotation of the blades 4, fast movingparticles, in particular, tin particles and gaseous and ion likeparticles traveling away from the source 6 can be trapped while EUVradiation, due to the speed of light, can travel generally unhindered bythe blades 4.

The foil trap 1 thus functions as a contamination barrier to trapcontaminant material coming from the radiation source 6. Typically, theblades 4 are arranged at a distance of 0.3-5 mm apart and have agenerally rectangular form. Advantageously, the source 6 is positionedat an intersection of extended planes through the plurality of blades 4which define an optical center of the contamination barrier 5, which inFIG. 2 coincides with the rotation shaft 3 of the foil trap 1. For anideal point like EUV source 6 at this center, radiation would pass in adirection generally parallel to an orientation of the blades 4. Thus,shielding of EUV radiation is low and only takes place over a thicknessof the blade (which, in an embodiment, is accordingly kept small withoutcompromising mechanical integrity). A typical value of the thickness ofthe blade can be about 100 microns, which may result in a shielding ofabout 10 percent of the radiation.

FIG. 3 shows a schematic view of an axial cross-sectional view of arotating foil trap such as illustrated in FIG. 2. In particular, FIG. 3shows a central rotating shaft 3 which is mounted in a bearing structure8, which comprises, in the context of this embodiment, two gas bearings9 enclosing the shaft 3. The gas bearings 9 are operated in a vacuumenvironment, which poses special constraints on the gap 10 between theshaft 3 and the bearings 9. For example, the gap 10 should be very smallin order not to pose problems for the vacuum environment. Thus, reducingor minimizing eccentric movements of the shaft 3 should allow for asmaller gap 10, which would be beneficial to operation in a vacuumenvironment.

In FIG. 3, a modeled representation of an axis of inertia 11 isschematically indicated. FIG. 4 shows a schematic lateralcross-sectional view of the rotating foil trap according to FIG. 3 alongline I-I, showing the axis of inertia viewed along an axial direction.The axis of inertia, for a rigid elongate body, can be represented bytwo off-centered masses 12 at the respective ends of the axis, which areschematically illustrated as not coincident with a central (geometrical)axis of rotation 13.

To reduce or eliminate the eccentricity of the axis of inertia, one ormore eccentric mass elements (see FIG. 6) may be provided in at leasttwo planes 14 lateral the central axis of rotation 13. In an embodiment,the one or more corrective masses are provided in the same planes 14 asis measured (as discussed below), but other lateral planes are possiblewhen taking into account the specific geometry of the blade structure 2.

In an embodiment, the imbalance of the blade structure 2 is measured bytwo force sensors 15 which sense a force in a single dimension, in twoplanes 14 spaced apart from each other along the central axis ofrotation 13. The blade structure 2 is mounted rigidly in anotherdirection than the measuring direction (see FIG. 5). The force sensors15 thus measure an eccentric displacement of the blade structure 2 in alateral plane. In an embodiment, also in this plane, one or moreeccentric mass elements (see FIG. 6) are provided relative to thecentral axis of rotation 13 in order to avoid recalculation necessaryfor geometric deviations.

The shaft 3 further comprises a coupling element 16 connecting the shaftpart that is borne by the gas bearings 9 to the shaft part 17 to whichthe blades 4 are mounted. The coupling element 16 is formed of amaterial having a relatively low thermal conductivity compared to thematerial of the shaft 3, which, due to the relevant operatingtemperature range of 800-1200° C., may be a molybdenum alloy. A suitablematerial for the coupling element 16 to provide the thermal isolation istantalum, because, for example, the coefficients of thermal expansion ofthe shaft 3 and the coupling element 16 would then almost match (5 μ/m-Kfor molybdenum while 6 μ/m-K for tantalum).

FIG. 5 shows schematically an exploded view of a balancing unit 18according to an embodiment of the invention. The balancing unit can beused as a test setup to test the balancing properties of the foil trap,in particular, of the blade structure 2. The blade structure 2(including the gas bearing structure 8) may be taken out of theoperative environment of the foil trap and inserted in the setup 18 asillustrated in FIG. 5. Due to the fragility of the blade structure 2,the balancing procedure should be carried out in vacuum conditions,therefore the balancing unit would be operated in a vacuum chamber (notshown). Furthermore, the (initial) test rotation frequency is usuallymuch lower, typically about 20-50 Hz, than the operating rotationfrequency of the foil trap.

In a balanced condition, the rotation frequency of the blade structure 2may be typically a factor ten higher. As discussed with reference toFIG. 2, force sensors 15 are present to sense a force variation in onedirection in order to provide a direct measurement of the imbalance ofthe blade structure 2. Alternatively, other imbalance sensing methodsmay be used, in particular, a displacement sensor which can becontactless or the like. The gas bearing structure 8 is stabilized byleaf springs 19 mounted in a rigid frame 20, which effectively limit thefreedom of movement to a single direction. In addition, a rotationsensor is present (not shown) to sense a rotation angle of the bladestructure 2. Based on the sensed force variations from the force sensors15 and the rotation angle from the rotation sensor, a calculating unit(not shown) calculates a location to add one or more eccentric masselement to the blade structure as well as the amount of mass that shouldbe added.

FIG. 6 shows a practical embodiment that may be used for automaticadjustment of the imbalance that is measured. This embodiment allows fora continuous operation in an operative environment of the foil trap 1.In particular, when a blade is accidentally lost or deformed in theblade structure 2, or if an imbalance is caused by an uneven spread ofcolliding debris from the radiation source 6, balance may be restored byadjustment of the balance weights.

In addition to the imbalance measurement sensors 15 detailed withrespect to FIGS. 2 and 5, an adjustment unit 21 may be provided toprovide automatic adjustment of one or more eccentric mass elements inresponse to an imbalance signal caused by an eccentricity schematicallyillustrated by item 22. Typically, along the central axis of rotation13, at least two of these adjustment units 21 may be used to balance theelongated geometrical form of the foil trap 1. The adjustment unit 21,in an embodiment, comprises a pair of rotatable mass elements 23, whichmay be positioned freely relative to the shaft 3 and provide aneffective eccentric mass that is adjustable relative to the central axisof rotation 13. Additionally or alternatively, other types of providingone or more (effective) eccentric mass elements are feasible, includingadding, shifting and/or removing mass from or attached to the rotatableshaft 3. For non-automatic adjustment, such as using the balancing unitdetailed in FIG. 5, a way of balancing is to provide eccentric boreholes in the rotatable shaft. In na embodiment, an aspect of theautomatic balancing unit or other balancing mechanism is that it mayallow for creation of a temporary imbalance, the temporary imbalancecausing vibrations that may be effective in cleaning cycles whencleaning the barrier 1.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A rotatable contamination barrier for use with an EUV radiationsystem, the barrier comprising: a blade structure configured to trapcontaminant material coming from a radiation source; a bearingstructure, coupled to a static frame, configured to rotatably bear theblade structure; and an eccentric mass element displaced relative to acentral axis of rotation to balance the blade structure in the bearingstructure.
 2. The barrier of claim 1, wherein a plurality of eccentricmass elements are provided in at least two planes displaced laterallyalong the central axis of rotation.
 3. The barrier of claim 1, whereinthe eccentric mass element is displaceable relative to the central axisof rotation.
 4. The barrier of claim 3, further comprising: a imbalancesensor configured to provide a signal of a sensed imbalance of thecontamination barrier in the bearing structure; and an adjustment unitconfigured to automatically adjust the eccentric mass element inresponse to the signal.
 5. The barrier of claim 4, wherein the imbalancesensor unit is configured to measure an eccentric displacement of theblade structure in a lateral plane that is the same as or parallel tothe lateral plane wherein the eccentric mass element is provided.
 6. Thebarrier of claim 1, wherein the bearing structure is configured to beoperated in a vacuum environment and comprises a gas bearing.
 7. Thebarrier of claim 1, wherein the blade structure comprises a rotatableshaft and a plurality of closely packed blades mounted to the rotatableshaft, the blades radially oriented relative to the rotatable shaft. 8.The barrier of claim 7, wherein the eccentric mass element comprises anaddition of mass to the rotatable shaft, a shift of mass on or in therotatable shaft, removal of mass from the rotatable shaft, or anycombination of the foregoing.
 9. The barrier of claim 7, wherein therotatable shaft is thermally stabilized against thermal energy impartedon the plurality of blades by EUV radiation and/or debris.
 10. Thebarrier of claim 9, wherein the rotating shaft comprises a thermallystabilizing coupling element configured to couple a shaft part that isborne in the bearing structure and a shaft part that provides a mount tothe blade structure.
 11. The barrier of claim 10, wherein the shaft partborne in the bearing structure comprises an alloy comprising molybdenumand wherein the coupling element is comprised of an alloy comprisingtantalum.
 12. A balancing unit to balance a rotatable contaminationbarrier, the unit comprising: a bearing structure configured to bear ablade structure of the rotatable contamination barrier; a imbalancesensor unit configured to provide a signal of a sensed imbalance of theblade structure in the bearing structure during rotation of the bladestructure; and a calculating unit configured to calculate a location toprovide one or more eccentric mass elements on or to the blade structureand the amount of such mass, the calculating unit communicativelycoupled to the imbalance sensor.
 13. The balancing unit of claim 12,wherein the calculating unit is configured to calculate a location ofthe one or more eccentric mass elements in a lateral plane that is thesame as or parallel to a lateral plane wherein the imbalance sensor unitsenses the imbalance.
 14. The balancing unit of claim 13, wherein thebearing structure is configured to be mounted in a vacuum environmentand comprises a gas bearing.
 15. The balancing unit of claim 12, whereinthe imbalance sensor unit comprises a plurality of force sensorsconfigured to measure a force exerted on the bearing structure by theblade structure when rotating.
 16. A method of balancing a rotatablecontamination barrier for use in an EUV radiation system, comprising:bearing a blade structure in a bearing structure provided in a vacuumenvironment; sensing an imbalance of the blade structure in the bearingstructure during rotation of the blade structure; and calculating, basedon the sensed imbalance, a location to provide an eccentric mass elementon or to the blade structure and an amount of such mass.
 17. The methodof claim 16, further comprising: providing the blade structure with aneccentric mass element to balance the blade structure in the bearingstructure; and automatically adjusting the eccentric mass element inresponse to the imbalance signal.
 18. A method of cleaning a rotatablecontamination barrier for use with an EUV radiation system, comprising:adjusting an eccentric mass element of the contamination barrier toprovide an imbalance; and rotating the contamination barrier with theimbalance to shake clean the contamination barrier.
 19. A lithographicapparatus, comprising: a rotatable contamination barrier configured toreceive a beam of radiation, the contamination barrier comprising ablade structure configured to trap contaminant material coming from aradiation source, a bearing structure, coupled to a static frame,configured to rotatably bear the blade structure, and an eccentric masselement displaced relative to a central axis of rotation to balance theblade structure in the bearing structure; an illumination systemconfigured to condition the radiation beam; a support constructed tosupport a patterning device, the patterning device being capable ofimparting the radiation beam with a pattern in its cross-section to forma patterned radiation beam; a substrate table constructed to hold asubstrate; and a projection system configured to project the patternedradiation beam onto a target portion of the substrate.